A brief description of carriers will be given hereinbelow.
A user may perform modulation on the amplitude, frequency, and/or phase of a sine wave or a periodic pulse wave to include information which is desired to be transmitted. At this time, the sine wave or pulse wave serving to convey information is called a carrier.
A method for modulating a carrier includes a single-carrier modulation (SCM) scheme and a multi-carrier modulation (MCM) scheme. The SCM scheme performs modulation such that all information is carried on a single carrier.
The MCM scheme divides an entire bandwidth channel of one carrier into subchannels having multiple narrow bandwidths and transmits multiple narrowband subcarriers through respective subchannels.
When using the MCM scheme, each subchannel may approximate to a flat channel due to a narrow bandwidth. A user may compensate for distortion of a channel using a simple equalizer. The MCM scheme may be implemented at a high speed using fast Fourier transform (FFT). Namely, the MCM scheme is favorable during high-rate data transmission as compared to the SCM scheme.
As the capabilities of a base station and/or a terminal have been developed, a frequency bandwidth which can be provided or used by the base station and/or the terminal has been enlarged. Accordingly, in the embodiments of the present invention, a multi-carrier system supporting broadband by aggregating one or more carriers is proposed.
Specifically, the multi-carrier system, which will be described hereafter, uses carriers by aggregating one or more carriers, unlike the afore-mentioned MCM scheme which uses carriers by segregating one carrier.
To efficiently use multiple bands or multiple carriers, a technique in which one medium access control (MAC) entity manages multiple carriers (e.g., multiple frequency allocations (FAs)) has been proposed.
FIGS. 1a and 1b illustrate methods for transmitting and receiving signals based on a multi-band radio frequency (RF).
In FIGS. 1a and 1b, one MAC layer in each of a transmitting end and a receiving end may manage multiple carriers to efficiently use the multiple carriers. To effectively transmit and receive the multiple carriers, it is assumed that both the transmitting end and the receiving end can transmit and receive the multiple carriers. Since frequency carriers managed by one MAC layer do not need to be contiguous, the above method enables flexible resource management. More specifically, the frequency carriers may have contiguous aggregation or non-contiguous aggregation.
In FIGS. 1a and 1b, PHY 0, PHY 1, . . . , PHY n-2, and PHY n-1 represent multiple bands and each of the bands may have an FA size allocated for a specific service according to a predetermined frequency policy. For example, PHY 0 (RF carrier 0) may have an FA size allocated for a general FM radio broadcast and PHY 1 (RF carrier 1) may have an FA size allocated for cellular phone communication.
Although each frequency band may have a different FA size depending on the characteristics thereof, it is assumed in the following description that each FA has a size of A MHz for convenience of explanation. Each FA may be represented by a carrier frequency that enables a baseband signal to be used in each frequency band. Thus, in the following description, each FA will be referred to as a “carrier frequency band” or will simply be referred to as a “carrier” representing each carrier frequency band unless such use causes confusion. As in the recent 3rd generation partnership project (3GPP) long term evolution-advanced (LTE-A), the carrier may also be referred to as a “component carrier” to discriminate it from a subcarrier used in the multi-carrier system.
From this aspect, the “multi-band” scheme may also be referred to as a “multi-carrier” scheme or a “carrier aggregation” scheme.
To transmit signals through multiple bands as shown in FIG. 1a and to receive signals through multiple bands as shown in FIG. 1b, a transmitter and a receiver need to include an RF module for transmitting and receiving signals through multiple bands. In FIGS. 1a and 1b, a method of configuring the “MAC” is determined by a base station, regardless of downlink or uplink.
In brief, the above scheme is a technology in which one MAC entity (hereinafter, referred to as a “MAC” unless such use causes confusion) manages and operates a plurality of RF carriers, thereby transmitting and receiving signals. The RF carriers managed by one MAC do not need to be contiguous. Accordingly, this technology has an advantage of high flexibility in terms of resource management.
FIG. 2 illustrates an example of a method for allocating frequencies in a multi-carrier system.
In FIG. 2, bands FA 0 to FA 7 can be managed by RF carriers RF 0 to RF 7. In the example of FIG. 2, it is assumed that the bands FA 0, FA 2, FA 3, FA 6, and FA 7 have already been allocated to specific existing communication services. It is also assumed that available RF carriers RF 1 (FA 1), RF 4 (FA 4), and RF 5 (FA 5) can be efficiently managed by one MAC (MAC #5). Here, since the RF carriers managed by one MAC need not to be contiguous as described above, it is possible to more efficiently manage frequency resources.
In the case of downlink, the following base station/terminal scenario exemplifies the concept of the above-described multi-band support scheme or carrier aggregation support scheme.
FIG. 3 illustrates an example of a scenario in which one base station communicates with a plurality of terminals in a multi-band support scheme.
In FIG. 3, it is assumed that terminals (mobile station (MSs) or user equipments (UEs)) 0, 1, and 2 have been multiplexed. A base station (BS) (or node-B) 0 transmits signals through frequency bands managed by RF carriers RF 0 and RF 1. It is also assumed that the terminal 0 is capable of receiving only the RF carrier RF 0, the terminal 1 is capable of receiving both the RF carriers RF 0 and RF 1, and the terminal 2 is capable of receiving all the RF carriers RF 0, RF 1, and RF 2. Here, since the base station transmits only the RF carriers RF 0 and RF 1, the terminal 2 receives signals of only the RF carriers RF 0 and RF 1.
However, the above multi-band based communication scheme has only been conceptually defined and can be interpreted as further allocating FA only as needed. Therefore, it is necessary to specify in more detail a multiplexing method or a signal transmitting/receiving method, for more efficient and higher performance.
Further, since channel coding or multiplexing is generally performed with respect to each frequency band as described above, a diversity gain or multiplexing gain may be limited.